I suspect you are misunderstanding the use case here. If I am understanding you correctly, you are proposing solution using single phone. As such, only one person can have the phone with them at any given time.
Also, a dumb phone doesn't really help with the need for SMS and MMS.
Update: I had indeed misunderstood and the proposal was to use a cloned SIM and two phones with dual SIM support. However, as discussed deeper in the thread this still seems to have flaws.
Also, a dumb phone doesn't really help with the need for SMS and MMS.
Update: I had indeed misunderstood and the proposal was to use a cloned SIM and two phones with dual SIM support. However, as discussed deeper in the thread this still seems to have flaws.